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Ocean bottom pressure pB is an important oceanic variable that is dynamically related to the abyssal ocean circulation through geostrophy. In this study we examine the seasonal pB variability in the North Pacific Ocean by analyzing satellite gravimetric observations from the GRACE program and a data-assimilated ocean-state estimate from ECCOv4. The seasonal pB variability is characterized by alternations of low and high anomalies among three regions, the subpolar and subtropical basins as well as the equatorial region. A linear two-layer wind-driven model is used to examine forcing mechanisms and topographic effects on seasonal pB variations. The model control run, which uses a realistic topography, is able to simulate a basinwide seasonal pB variability that is remarkably similar to that from GRACE and ECCOv4. Since the model is driven by wind stress alone, the good model–data agreement indicates that wind stress is the leading forcing for seasonal changes in pB. An additional model simulation was conducted by setting the water depth uniformly at 5000 m. The magnitude of seasonal pB anomaly is amplified significantly in the flat-bottom simulation as compared with that in the control run. The difference can be explained in terms of the topographic Sverdrup balance. In addition, the spatial pattern of the seasonal pB variability is also profoundly affected by topography especially on continental margins, ridges, and trenches. Such differences are due to topographic effects on the propagation pathways of Rossby waves. [ABSTRACT FROM AUTHOR]

Previous in situ observations have suggested that bottom water temperature variations in shelf seas can drive significant ocean bottom heat flux (BHF) by heat conduction. The BHF-driven bottom water temperature variations, however, have been overlooked in ocean general circulation models. In this study, we established a sea-sediment fully coupled model through incorporating the BHF. The coupled model included a sediment temperature module/model, and the BHF was calculated based on the sediment heat content variations. Meanwhile, we applied temporally varying BHF in the calculation of the bottom water temperature, which further determined the sediment temperature. The two-way coupled BHF process presents a more complete and physically reasonable heat budget in the ocean model and a synchronously varying sediment temperature profile. The coupled model was validated using a one-dimensional test case, and then it was applied in a domain covering the Bohai and Yellow Seas. The results suggest that when a strong thermocline exists, the BHF can change the bottom water temperature by more than 1°C because the effects of the BHF are limited to within a shallow bottom layer. However, when the water column is well mixed, the BHF changes the temperature of the entire water column, and the heat transported across the bottom boundary is ventilated to the atmosphere. Thus, the BHF has less effect on water temperature and may directly affect air–sea heat flux. The sea-sediment interactions dampen the amplitude of the bottom water temperature variations, which we propose calling the seabed dampening ocean heat content variation mechanism (SDH). [ABSTRACT FROM AUTHOR]

The abyssal southwest Pacific basin has warmed significantly between 1992 and 2017, consistent with warming along the bottom limb of the meridional overturning circulation seen throughout the global oceans. Here we present a framework for assessing the abyssal heat budget that includes the time-dependent unsteady effects of decadal warming and direct and indirect estimates of diapycnal mixing from microscale temperature measurements and finescale parameterizations. The unsteady terms estimated from the decadal warming rate are shown to be within a factor of 3 of the steady-state terms in the abyssal heat budget for the coldest portion of the water column and therefore cannot be ignored. We show that a reduction in the lateral heat flux for the coldest temperature classes compensated by an increase in warmer waters advected into the basin has important implications for the heat balance and diffusive heat fluxes in the basin. Vertical diffusive heat fluxes are estimated in different ways: using the newly available CTD-mounted microscale temperature measurements, a finescale strain parameterization, and a vertical kinetic energy parameterization from data along the P06 transect along 32.5°S. The unsteady-state abyssal heat budget for the basin shows closure within error estimates, demonstrating that (i) unsteady terms have become consequential for the heat balance in the isotherms closest to the ocean bottom and (ii) direct and indirect estimates from full-depth GO-SHIP hydrographic transects averaged over similarly large spatial and temporal scales can capture the basin-averaged abyssal mixing needed to close the deep overturning circulation. Significance Statement: The deep overturning circulation of the ocean is driven by northward flow of cold waters formed in the Southern Ocean and diffusion-driven upwelling from oceanic turbulence, both of which are difficult to measure and quantify accurately. This study aims to reconcile different methods of measuring the overturning in a large ocean basin by 1) inferring mixing required to sustain the overturning using long-term observations of inflow and outflow heat fluxes from a time-dependent heat budget and 2) direct/indirect observations of mixing within the basin. While our results show consistency of the basin-averaged mixing estimates from several different methods, this study also highlights the need to make sustained oceanic observations of mixing and circulation to further reduce uncertainty in the estimates. [ABSTRACT FROM AUTHOR]

The World Ocean is estimated to store more than 90% of the excess energy resulting from man-made greenhouse gas-driven radiative forcing as heat. Uncertainties of this estimate are related to undersampling of the subpolar and polar regions and of the depths below 2000 m. Here we present measurements from the Weddell Sea that cover the whole water column down to the sea floor, taken by the same accurate method at locations revisited every few years since 1989. Our results show widespread warming with similar long-term temperature trends below 700-m depth at all sampling sites. The mean heating rate below 2000m exceeds that of the global ocean by a factor of about 5. Salinity tends to increase--in contrast to other Southern Ocean regions--at most sites and depths below 700 m, but nowhere strongly enough to fully compensate for the warming effect on seawater density, which hence shows a general decrease. In the top 700m neither temperature nor salinity shows clear trends. A closer look at the vertical distribution of changes along an approximately zonal and a meridional section across the Weddell Gyre reveals that the strongest vertically coherent warming is observed at the flanks of the gyre over the deep continental slopes and at its northern edge where the gyre connects to the Antarctic Circumpolar Current (ACC). Most likely, the warming of the interior Weddell Sea is driven by changes of the Weddell Gyre strength and its interaction with the ACC. [ABSTRACT FROM AUTHOR]

Deployment of Deep Argo regional pilot arrays is underway as a step toward a global array of 1250 surface-to-bottom profiling floats embedded in the upper-ocean (2000 m) Argo Program. Of the 80 active Deep Argo floats as of July 2019, 55 are Deep Sounding Oceanographic Lagrangian Observer (SOLO) 6000-m instruments, and the rest are composed of three additional models profiling to either 4000 or 6000 m. Early success of the Deep SOLO is owed partly to its evolution from the Core Argo SOLO-II. Here, Deep SOLO design choices are described, including the spherical glass pressure housing, the hydraulics system, and the passive bottom detection system. Operation of Deep SOLO is flexible, with the mission parameters being adjustable from shore via Iridium communications. Long lifetime is a key element in sustaining a global array, and Deep SOLO combines a long battery life of over 200 cycles to 6000 m with robust operation and a low failure rate. The scientific value of Deep SOLO is illustrated, including examples of its ability (i) to observe large-scale spatial and temporal variability in deep ocean temperature and salinity, (ii) to sample newly formed water masses year-round and within a few meters of the sea floor, and (iii) to explore the poorly known abyssal velocity field and deep circulation of the World Ocean. Deep SOLO's full-depth range and its potential for global coverage are critical attributes for complementing the Core Argo Program and achieving these objectives. [ABSTRACT FROM AUTHOR]

The large-scale circulation of the abyssal ocean is enabled by small-scale diapycnal mixing, which observations suggest is strongly enhanced toward the ocean bottom, where the breaking of internal tides and lee waves is most vigorous. As discussed recently, bottom-intensified mixing induces a pattern of near-bottom up- and downwelling that is quite different from the traditionally assumed widespread upwelling. Here the consequences of bottom-intensified mixing for the horizontal circulation of the abyssal ocean are explored by considering planetary geostrophic dynamics in an idealized “bathtub geometry.” Up- and downwelling layers develop on bottom slopes as expected, and these layers are well described by boundary layer theory. The basin-scale circulation is driven by flows in and out of these boundary layers at the base of the sloping topography, which creates primarily zonal currents in the interior and a net meridional exchange along western boundaries. The rate of the net overturning is controlled by the up- and downslope transports in boundary layers on slopes and can be predicted with boundary layer theory. [ABSTRACT FROM AUTHOR]

The dense water outflow from the Antarctic continental shelf is closely associated with the strength and position of the Antarctic Slope Front. This study explores the short-term and spatial variability of the Antarctic Slope Front system and the mechanisms that regulate cross-slope exchange using highly temporally and spatially resolved measurements from three ocean gliders deployed in 2012. The 22 sections along the eastern Antarctic Peninsula and west of the South Orkney Islands are grouped regionally and composited by isobaths. There is consistency in the front position around the Powell Basin, varying mostly between the 500- and 800-m isobaths. In most of the study area the flow is bottom intensified. The along-slope transport of the Antarctic Slope Current (upper 1000 m) varies between 0.2 and 5.9 Sv (1 Sv #8801; 106m3 s-1) and does not exhibit a regional pattern. The magnitude of the velocity field shows substantial variability, up to twice its mean value. Higher eddy kinetic energy (0.003m2 s-2) is observed in sections with dense water, possibly because of baroclinic instabilities in the bottom layer. Distributions of potential vorticity show an increase toward the shelf along isopycnals and also in the dense water layer. Glider sections located west of the South Orkney Islands indicate a northward direction of the flow associated with the Weddell Front, which differs from previous estimates of the mean circulation. This study provides some of the first observational confirmation of the high-frequency variability associated with an active eddy field that has been suggested by recent numerical simulations in this region. [ABSTRACT FROM AUTHOR]

Weddell Polynya transport mechanisms in the deep and abyssal oceans are examined in the NOAA Geophysical Fluid Dynamics Laboratory's (GFDL) coupled climate model CM2G. During an 1820-yr-long integration of the model, polynyas are forced every 29 years in the Weddell Sea via an increase in the diapycnal diffusivity. Composites of the events are used to examine the mechanisms responsible for transporting polynya signals away from the Weddell Sea. Polynya signal transport is governed by two dynamical mechanisms that act on different time scales and spread at different rates. Large-scale waves, such as Kelvin and planetary and topographic Rossby waves, propagate the polynya signal rapidly, on interannual-to-decadal time scales, while advection transports the signal more slowly, on decadal-to-centennial time scales. Despite their different spreading rates, these mechanisms can act contemporaneously, and it is often their combined effect that governs the property changes in the global deep and abyssal oceans. Both waves and advection cause temperature changes on isobaths. In the deep Atlantic, advection accounts for ,15% of the total temperature change in the model, indicating that waves are strongly dominant there. Elsewhere, waves are still the stronger contributor, but advection accounts for 20%-40% of the total temperature change. [ABSTRACT FROM AUTHOR]

This is Part II of a two-part theoretical study into the midlatitude-cross-equatorial dynamics of a deep western boundary current (DWBC) in an idealized meridionally aligned, differentially rotating ocean basin with zonally varying parabolic bottom topography. Part I determined the midlatitude flow across the planetary vorticity gradient and the dynamics of the DWBC as it begins to enter the equatorial region in the 'intermediate equatorial region.' Part II determines the nonlinear dynamics of the DWBC as it flows across the basin along the equator in the 'inner equatorial region.' The large-scale structure of the flow within the inner equatorial region corresponds to a zonally aligned nonlinear stationary planetary wave pattern that meanders about the equator in which the flow exits the equatorial region on the eastern side of the basin. In addition to numerically determining the pathlines for the large-scale equatorial flow, an approximate nonlinear model is introduced for which an analytical solution can be obtained for the nonlinear planetary wave along the equator. If the DWBC exits the equatorial region into the opposite hemisphere from its source hemisphere, the characteristic curves associated with the flow must necessarily intersect within the inner equatorial region. It is in the regions of intersecting characteristics that dissipation makes a leading-order contribution to the dynamics and induces the requisite potential vorticity adjustment permitting the cross-equatorial flow of a DWBC that is in planetary geostrophic dynamical balance in midlatitudes. [ABSTRACT FROM AUTHOR]

A comprehensive theoretical study of the nonlinear hemispheric-scale midlatitude and cross-equatorial steady-state dynamics of a grounded deep western boundary current is given. The domain considered is an idealized differentially rotating, meridionally aligned basin with zonally varying parabolic bottom topography so that the model ocean shallows on both the western and eastern sides of the basin. Away from the equator, the flow is governed by nonlinear planetary geostrophic dynamics on sloping topography in which the potential vorticity equation can be explicitly solved. As the flow enters the equatorial region, it speeds up and becomes increasingly nonlinear and passes through two distinguished inertial layers referred to as the 'intermediate' and 'inner' inertial equatorial boundary layers, respectively. The flow in the intermediate equatorial region is shown to accelerate and turn eastward, forming a narrow equatorial jet. The qualitative properties of the solution presented are consistent with the known dynamical characteristics of the deep western boundary currents as they flow from the midlatitudes into the tropics. The predominately zonal flow across the ocean basin in the inner equatorial region (and its exit from the equatorial region) is determined in Part II of this study. [ABSTRACT FROM AUTHOR]

The meridional overturning circulation (MOC) is composed of interconnected overturning cells that transport cold dense abyssal waters formed at high latitudes back to the surface. Turbulent diapycnal mixing plays a primary role in setting the rate and patterns of the various overturning cells that constitute the MOC. The focus of the analyses in this paper will be on the influence of sharp vertical variations in mixing on the MOC and ocean stratification. Mixing is enhanced close to the ocean bottom topography where internal waves generated by the interaction of tides and geostrophic motions with topography break. It is shown that the sharp vertical variations in mixing lead to the formation of three layers with different dynamical balances governing meridional flow. Specifically, an abyssal bottom boundary layer forms above the ocean floor where mixing is largest and hosts the northward transport of the heaviest waters from the southern channel to the closed basins. A deep layer forms above the bottom layer in which the upwelled waters return south. A third adiabatic layer lies above the other two. While the adiabatic layer has been studied in detail in recent years, the deep and bottom layers are less appreciated. It is shown that the bottom layer, which is not resolved or allowed for in most idealized models, must be present to satisfy the no flux boundary condition at the ocean floor and that its thickness is set by the vertical profile of mixing. The deep layer spans a considerable depth range of the ocean within which the stratification scale is set by mixing, in line with the classic view of Munk in 1966. [ABSTRACT FROM AUTHOR]

The ocean surface vorticity budget around the Hawaiian Islands is examined using an 18-month model reanalysis generated using four-dimensional variational state estimation with all available observations (satellite, in situ, and high-frequency radio). To better resolve the ocean surface currents and reduce the representation error of the radio-measured surface currents, this study developed a new vertical scheme for the Regional Ocean Modeling System. A new, detailed description of the ocean surface vorticity is created, revealing a region dominated by cyclonic (anticyclonic) vorticity to the north (south) of the mean position of the Hawaii Lee Countercurrent. Advection of vorticity is the primary process that transports the vorticity generated in the lee of the islands by the wind wake. In this island lee, the zonal wavenumber spectra show a cascade of vorticity/energy from the submesoscale toward the larger scales. Latitudinal differences in the advection of vorticity spectra indicate the propagation of a first baroclinic mode Rossby wave in the region dominated by anticyclonic vorticity to the west of the island of Hawaii, while the region dominated by cyclonic vorticity is disrupted by the wake of the smaller islands. [ABSTRACT FROM AUTHOR]

At several locations around Antarctica, dense water is formed as a result of intense sea ice formation. When this dense water becomes sufficiently denser than the surrounding water, it descends the continental slope and forms Antarctic Bottom Water (AABW). This study presents the AABW formation off the coast of Cape Darnley [Cape Darnley Bottom Water (CDBW)] in East Antarctica, using a nonhydrostatic model. The model is forced for 8 months by a temporally uniform surface salt flux (because of sea ice formation) estimated from Advanced Microwave Scanning Radiometer for Earth Observing System (EOS; AMSR-E) data and a heat budget calculation. The authors reproduce AABW formation and associated periodic downslope flows of dense water. Descending pathways of dense water are largely determined by the topography; most dense water flows into depressions on the continental shelf, advects onto the continental slope, and is steered downslope to greater depths by the canyons. Intense sea ice formation is the most important factor in the formation of AABW off Cape Darnley, and the existence of depressions is of only minor importance for the flux of CDBW. The mechanism responsible for the periodic downslope flow of dense water is further analyzed using an idealized model setup. The period of dense water outflow is regulated primarily by the topographic beta effect. [ABSTRACT FROM AUTHOR]

The classic baroclinic, wind-driven, double gyre problem is considered over a range of deformation radii, wind stress amplitudes, and bottom friction coefficients with the aim of better understanding the transfer of energy across scales. In this β-plane basin setting, significant differences are found with respect to classic studies of geostrophic turbulence, which generally assume zonal periodicity and for which the β term does not play a direct role in the energy transfers. In a basin geometry, the β term can play a direct role in the transfers; for example, it can be the dominant term allowing for energy transfer between the basin scale and the baroclinic mesoscale. It is also found that barotropization of baroclinic energy forces the barotropic mode near scales at which bottom drag damps this mode. Associated with this, the barotropic, nonlinear, inverse energy cascade does not extend between mesoscale injection and large-scale dissipation wavenumbers, as is often assumed. Instead, it is part of a 'double cascade' of barotropic energy in which the nonlinear inverse cascade is nearly offset by a forward cascade associated with the β term. This is particularly evident in weak bottom drag simulations, for which a time eddy-mean decomposition of the flow reveals the double cascade to be associated with the eddy-only terms. [ABSTRACT FROM AUTHOR]

Analyses of large-scale (>750 km) ocean bottom pressure p b fields, derived from the Gravity Recovery and Climate Experiment (GRACE) and from an Estimating the Circulation & Climate of the Ocean (ECCO) state estimate, reveal enhanced interannual variability, partially connected to the Antarctic Oscillation, in regions of the Australian-Antarctic Basin and the Bellingshausen Basin, with p b magnitudes comparable to those of sea level and good correlation between the GRACE and ECCO p b series. Consistent with the theory of Gill and Niiler, the patterns of stronger p b variability are partly related to enhanced local wind curl forcing and weakened gradients in H/ f, where H is ocean depth and f is the Coriolis parameter. Despite weaker H/ f gradients, motions against them are sufficiently strong to play a role in balancing the local wind input. Topographic effects are as or more important than changes in f. Additionally, and contrary to the dominance of barotropic processes at subannual time scales, baroclinic effects are not negligible when balancing wind input at periods of a few years. Results highlight the emerging capability to accurately observe and estimate interannual changes in large-scale p b over the Southern Ocean, with implications for the interpretation of low-frequency variability in sea level in terms of steric height and heat content. [ABSTRACT FROM AUTHOR]

The problem is posed and solved for the oceanic surface boundary layer in the presence of wind stress, stable density stratification, equilibrium wind-waves, and remotely generated swell-waves. The addition of swell causes an amplification of the Lagrangian-mean current and rotation toward the swell-wave direction, a fattening of the Ekman velocity spiral and associated vertical Reynolds stress profile, an amplification of the inertial current response, an enhancement of turbulent variance and buoyancy entrainment rate from the pycnocline, and-for very large swell-an upscaling of the coherent Langmuir circulation patterns. Implications are discussed for the parameterization of Langmuir turbulence influences on the mean current profile and the material entrainment rate in oceanic circulation models. In particular, even though the turbulent kinetic energy monotonically increases with wave amplitude inversely expressed by the turbulent Langmuir number La, the Lagrangian shear eddy viscosity profile κ L( z) is a nonmonotonic function of La, first increasing with increasing wave amplitude up to approximately the wind-wave equilibrium level, then decreasing with additional swell-wave amplitude. In contrast, the pycnocline entrainment rate is a monotonic function ~La−2. [ABSTRACT FROM AUTHOR]

Tropical instability vortices (TIVs) in the equatorial Pacific exhibit energetic horizontal and vertical circulation characterized by regions of high Rossby number and low Richardson number. Their strong anticyclonic vorticity and vertical shear can influence the broader-scale circulation by driving lateral mixing and vertical exchange between the ocean surface and interior. The authors use a set of nested high-resolution simulations of the equatorial Pacific, with a finest grid size of 3 km, to examine the vortex dynamics associated with TIV core water formation. TIV cores are characterized by low values of the Ertel potential vorticity (PV) as the relative vorticity is anticyclonic with magnitude comparable to the local Coriolis parameter. A study of the variation of PV and other scalars along Lagrangian fluid parcel tracks entering the TIVs shows that the low-PV water in their cores is a mix of Equatorial Undercurrent (EUC) water and North Equatorial Counter Current (NECC) water. The EUC water is characterized by strong horizontal vorticity, and thus, the baroclinic component of the PV is nonnegligible and acts as a source for the anticyclonic vorticity of TIVs. This horizontal vorticity is tilted by an ageostrophic secondary circulation associated with strain-induced frontogenesis that tends to form along the path of the EUC water that enters the vortex. Frontogenesis disrupts the cyclogeostrophic balance of the frontal flow and drives differential vertical motions across the front. These results emphasize the role of submesoscale physics in the equatorial region, which are active when both the Rossby and Richardson numbers are O(1). [ABSTRACT FROM AUTHOR]

Mesoscale eddies play a major role in the transport of tracers in the ocean. Focusing on a sector in the east Pacific, the authors present estimates of eddy diffusivities derived from kinematic tracer simulations using satellite-observed velocity fields. Meridional diffusivities are diagnosed, and how they are related to eddy properties through the mixing length formulation of Ferrari and Nikurashin, which accounts for the suppression of diffusivity due to eddy propagation relative to the mean flow, is shown. The uniqueness of this study is that, through systematically varying the zonal-mean flow, a hypothetical 'unsuppressed' diffusivity is diagnosed. At a given latitude, the unsuppressed diffusivity occurs when the zonal-mean flow equals the eddy phase speed. This provides an independent estimate of eddy phase propagation, which agrees well with theoretical arguments. It is also shown that the unsuppressed diffusivity is predicted very well by classical mixing length theory, that is, that it is proportional to the rms eddy velocity times the observed eddy size, with a spatially constant mixing efficiency of 0.35. Then, the suppression factor is estimated and it is shown that it too can be understood quantitatively in terms of easily observed mean flow properties. The authors then extrapolate from these sector experiments to the global scale, making predictions for the global surface eddy diffusivity. Together with a prognostic equation for eddy kinetic energy and a theory explaining observed eddy sizes, these concepts could potentially be used in a closure for eddy diffusivities in coarse-resolution ocean climate models. [ABSTRACT FROM AUTHOR]

This study examines the dynamics of a buoyant river plume in upwelling-favorable winds, concentrating on the time after separation from the coast. A set of idealized numerical simulations is used to examine the effects of breaking surface gravity waves on plume structure and cross-shore dynamics. Inclusion of a wave-breaking parameterization in the two-equation turbulence submodel causes the plume to be thicker and narrower, and to propagate offshore more slowly, than a plume in a simulation with no wave breaking. In simulations that include wave breaking, the plume has much smaller vertical gradients of salinity and velocity than in the simulation without breaking. This leads to decreased importance of shear dispersion in the plumes with wave breaking. Much of the widening rate of the plume is explained by divergent Ekman velocities at the off- and onshore edges. Some aspects of plume evolution in all cases are predicted well by a simple theory based on a critical Richardson number and an infinitely deep ocean. However, because the initial plume in these simulations is in contact with the sea floor in the inner shelf, some details are poorly predicted, especially around the time that the plume separates from the coast. [ABSTRACT FROM AUTHOR]

Turbulence observations at the shoal-channel interface in South San Francisco Bay are described and analyzed in this work. Profiles of turbulent kinetic energy (TKE) shear production and dissipation rate are estimated from ADCP beam velocities over a nine-day period during winter 2009 when the estuary was partially stratified. The TKE shear production profiles are consistent with a tidally driven bottom boundary layer, except late in the ebb when shear production intensifies and the region of maximum production is located in the upper half of the water column. A simple scaling for the depth-averaged TKE shear production based on bed friction performs well except during this late ebb period when it fails to predict the peak in shear production, which suggests that a mechanism other than bottom drag drives turbulence dynamics during this period. The profiles of TKE dissipation rates also display a late ebb peak. A lateral circulation develops at the shoal-channel interface during this late ebb period, and its effects on turbulence dynamics are investigated. An analytical framework is developed to identify and quantify these effects. Four mechanisms through which the lateral circulation can impact stability are identified, and two are found to be important during the late ebb period in South San Francisco Bay: the straining of the lateral density and velocity gradients in the lateral circulation. These results suggest that the lateral circulation acts to reduce the gradient Richardson number and is driving the late ebb peak in turbulence production and dissipation. [ABSTRACT FROM AUTHOR]

The impact of small-scale topography on the ocean's dynamical balance is investigated by quantifying the rates at which internal wave drag extracts (angular) momentum and vorticity from the general circulation. The calculation exploits the recent advent of two near-global descriptions of topographic roughness on horizontal scales on the order of 1-10 km, which play a central role in the generation of internal lee waves by geostrophic flows impinging on topography and have been hitherto unresolved by bathymetric datasets and ocean general circulation models alike. It is found that, while internal wave drag is a minor contributor to the ocean's dynamical balance over much of the globe, it is a significant player in the dynamics of extensive areas of the ocean, most notably the Antarctic Circumpolar Current and several regions of enhanced small-scale topographic variance in the equatorial and Southern Hemisphere oceans. There, the contribution of internal wave drag to the ocean's (angular) momentum and vorticity balances is generally on the order of ten to a few tens of percent of the dominant source and sink terms in each dynamical budget, which are respectively associated with wind forcing and form drag by topography with horizontal scales from 500 to 1000 km. It is thus suggested that the representation of internal wave drag in general circulation models may lead to significant changes in the deep ocean circulation of those regions. A theoretical scaling is derived that captures the basic dependence of internal wave drag on topographic roughness and near-bottom flow speed for most oceanographically relevant regimes. [ABSTRACT FROM AUTHOR]

This study reveals the dynamics behind the intensified, downslope, cross-isobath transport over a widened shelf (narrowing downwave) in the northeastern South China Sea (NSCS) during a downwelling event. Utilizing a three-dimensional numerical model over an idealized NSCS shelf, this study identified the forcing process and derived an analytical understanding of the invoked dynamics for the intensified downslope cross-isobath transport. This study found that the transport was formed by an amplified geostrophic transport because of an increased positive along-isobath pressure gradient force (PGF), PY*, and by an enhanced bottom Ekman transport due to converging flow over the widened shelf. Based on the depth-integrated vorticity dynamics, PY* was linked with net frictional stress curl in the water column and the curl was primarily associated with the shear vorticity field induced by downwelling jet. The increased positive vorticity seaside of the jet provided positive PY* for the downslope geostrophic transport. The cross-isobath PGF, PX*, which was geostrophically formed and shaped by the spatially asymmetric isobaths of the widened shelf, determined the intensities of the jet and thus the associated shear vorticity that quantified the respective bottom friction transport and PY* for the intensified cross-isobath transport. It was found that the downslope transport over the widened shelf was strengthened in a stratified flow by the linked intensifications of barotropic PX* or the concurrent downwelling jet, shear vorticity field, PY*, and bottom friction because of an increase of volume flux upstream of the widened shelf. [ABSTRACT FROM AUTHOR]

Comparably little is known about the impact of down-front-propagating surface waves on the stability of submesoscale lateral fronts in the ocean surface mixed layer. In this investigation, the stability of lateral fronts in gradient-wind balance to two-dimensional (down-front invariant) disturbances is analyzed using the stratified, rotating Craik-Leibovich (CL) equations. Through the action of the CL vortex force, the surface waves fundamentally alter the superinertial, two-dimensional linear stability of these fronts, with the classical symmetric instability mode being replaced by a hybrid Langmuir circulationisymmetric mode. The hybrid mode is shown to exhibit much larger growth rates than the pure symmetric mode, to exist in a regime in which the vertical Richardson number is greater than 1, and to accomplish significant cross-isopycnal transport. Nonhydrostatic numerical simulations reveal that the nonlinear evolution of this hybrid instability mode can lead to rapid, that is, superinertial, vertical restratification of the mixed layer. Paradoxically, Langmuir circulation-generally viewed as a prominent vertical mixing mechanism in the upper ocean-may thus play a role in mixed layer restratification. [ABSTRACT FROM AUTHOR]

Subduction-the transport of fluid across the base of mixed layer-exchanges water masses and tracers between the ocean surface and interior. Eddies can affect subduction in a variety of ways. First, eddies shoal the mixed layer by restratifying water columns through baroclinic instabilities. Second, eddies induce an isopycnic transport that leads to the entrainment of warm waters and subduction of cold waters, which effectively counters the wind-driven overturning circulation. In this study, the authors use an idealized model to examine these two mechanisms by which eddies influence subduction and to discuss how eddy subduction may be better approximated using the concept of vertical transport streamfunction than the conventional meridional transport streamfunction. [ABSTRACT FROM AUTHOR]

An analysis of the Indian Ocean circulation and the Agulhas Current retroflection is carried out using a primitive equation model with simplified coastline and flat bottom. Four configurations with 0.25° and 0.1° horizontal resolution and in barotropic and baroclinic cases are considered. The wind stress is taken as control parameter to increase the inertia of the currents. The volume transport of the Indonesian Throughflow, Mozambique Channel, and Agulhas Current are found to increase linearly with the wind stress strength, and three nonlinear retroflection regimes are found. A viscous and an inertial regime had already been docu-mented, but a new turbulent regime appears at large wind stress amplitude. In this turbulent regime, the volume of Agulhas leakage reaches a plateau because of strong mesoscale variability and, in contrast to the other regimes, does not depend on the wind stress magnitude. The physical mechanism causing the plateau is shown to be associated with the cross-jet exchange of Indian Ocean water and water from the Antarctic Circumpolar Current. In the turbulent regime, the permeability of the Agulhas Return Current to material transport increases and the Indian Ocean water available for the Agulhas leakage decreases. [ABSTRACT FROM AUTHOR]

Velocity measurements from satellite-tracked surface drifters collected between 1994 and 2010 are used to map the surface circulation in the Solomon Sea, the last passageway for waters of subtropical origin flowing northward toward the equator, where they replenish the Pacific warm pool. Pseudo-Eulerian statistics of the drifter observations show a strong seasonal cycle in both the mean circulation and the eddy kinetic energy in the region. The circulation is characterized by a strong northward flow from June to November (the season of strong southeasterly trade winds over the Solomon Sea) and a mostly southward flow with increased variability from December to May (when the winds over the sea are weak). The seasonal velocity signal has the largest magnitude narrowly along the double western boundary formed by the eastern coastlines of New Guinea and the Solomon Islands, suggesting that direct wind driving with its much larger spatial scales is not the main influence. In addition, the surface circulation exhibits substantial interannual variability of magnitude comparable to that of the seasonal cycle with velocity and temperature anomalies consistent with changes in the western boundary current acting to compensate for the discharge and recharge of the Pacific warm pool during ENSO. [ABSTRACT FROM AUTHOR]

Insights into the probability distribution of ocean currents are important for various applications such as the chance to encounter extreme events, which may affect, for example, marine construction, and for estimating the energy that can be extracted from the ocean. In addition, for devising better parameterizations for submesoscale mixing, which present climate models cannot resolve, one should understand the velocity distribution and its relation to the various forcing of surface ocean circulation. Here, the authors investigate the probability distribution of surface currents from the Gulf of Eilat/Aqaba measured by high-frequency radar. Their results show that the distribution of ocean current speeds can be approximated by a Weibull distribution. Moreover, the authors demonstrate the existence of spatial variations of the scale and shape parameters of the Weibull distribution over a relatively small region of only a few kilometers. They use a simple surface Ekman layer model to investigate this spatial variability. They find that, when forced by local winds, this model does not reproduce the observations. The addition of Gaussian noise to the zonal and meridional components of the bottom geostrophic currents has only a slight effect on the surface current distribution. However, noise added to the components of the local wind (mimicking wind gusts) has a much greater effect on the distribution of surface currents, suggesting that wind spatial and temporal variability underlay the observed spatial variability of the parameters of the Weibull distribution. [ABSTRACT FROM AUTHOR]

The authors estimate water mass transformation rates resulting from surface buoyancy fluxes and interior diapycnal fluxes in the region south of 30°S in the Estimating the Circulation and Climate of the Ocean (ECCO) model-based state estimation and three free-running coupled climate models. The meridional transport of deep and intermediate waters across 30°S agrees well between models and observationally based estimates in the Atlantic Ocean but not in the Indian and Pacific, where the model-based estimates are much smaller. Associated with this, in the models about half the southward-flowing deep water is converted into lighter waters and half is converted to denser bottom waters, whereas the observationally based estimates convert most of the inflowing deep water to bottom waters. In the models, both Antarctic Intermediate Water (AAIW) and Antarctic Bottom Water (AABW) are formed primarily via an interior diapycnal transformation rather than being transformed at the surface via heat or freshwater fluxes. Given the small vertical diffusivity specified in the models in this region, the authors conclude that other processes such as cabbeling and thermobaricity must be playing an important role in water mass transformation. Finally, in the models, the largest contribution of the surface buoyancy fluxes in the Southern Ocean is to convert Upper Circumpolar Deep Water (UCDW) and AAIW into lighter Subantarctic Mode Water (SAMW). [ABSTRACT FROM AUTHOR]

A numerical simulation of the tropical Atlantic Ocean indicates that surface cooling in upwelling zones of the Gulf of Guinea is mostly due to vertical mixing. At the seasonal scale, the spatial structure and the time variability of the northern and southern branches of the South Equatorial Current (SEC), and of the Guinea Current, are correlated with the timing and distribution of turbulent heat fluxes in the Gulf of Guinea. Through modulation of the velocity shear at the subsurface, these surface currents control the vertical turbulent exchanges, bringing cold and nutrient-rich waters to the surface. This mechanism explains the seasonality and spatial distribution of surface chlorophyll concentrations better than the generally accepted hypothesis that thermocline movements control the nutrient flux. The position of the southern SEC explains why the cold tongue and high chlorophyll concentrations extend from the equator to 4°°S in the southeastern part of the basin. [ABSTRACT FROM AUTHOR]

A simple theoretical model of the deep stratification and meridional overturning circulation in an idealized single-basin ocean with a circumpolar channel is presented. The theory includes the effects of wind, eddies, and diapycnal mixing; predicts the deep stratification in terms of the surface forcing and other problem parameters; makes no assumption of zero residual circulation; and consistently accounts for the interaction between the circumpolar channel and the rest of the ocean. The theory shows that dynamics of the overturning circulation can be characterized by two limiting regimes, corresponding to weak and strong diapycnal mixing. The transition between the two regimes is described by a nondimensional number characterizing the strength of the diffusion-driven compared to the wind-driven overturning circulation. In the limit of weak diapycnal mixing, deep stratification throughout the ocean is produced by the effects of wind and eddies in a circumpolar channel and maintained even in the limit of vanishing diapycnal diffusivity and in a flat-bottomed ocean. The overturning circulation across the deep stratification is driven by the diapycnal mixing in the basin away from the channel but is sensitive, through changes in stratification, to the wind and eddies in the channel. In the limit of strong diapycnal mixing, deep stratification is primarily set by eddies in the channel and diapycnal mixing in the basin away from the channel, with the wind over the circumpolar channel playing a secondary role. Analytical solutions for the deep stratification and overturning circulation in the limit of weak diapycnal mixing and numerical solutions that span the regimes of weak to strong diapycnal mixing are presented. The theory is tested with a coarse-resolution ocean general circulation model configured in an idealized geometry. A series of experiments performed to examine the sensitivity of the deep stratification and the overturning circulation to variations in wind stress and diapycnal mixing compare well with predictions from the theory. [ABSTRACT FROM AUTHOR]

Measurements of turbulence are made in a weakly but variably stratified region of tidal straining in the eastern Irish Sea using turbulence sensors profiling vertically through the water column on the Fast Light Yo-yo (FLY) profiler and horizontally on the Autonomous Underwater Vehicle (AUV) Autosub. The tidal currents exceed 1 m s-1 at the location of the measurements in water of a depth of about 43.5 m, and result in turbulence extending from the seabed to the surface with a cycle period that is half that of the tides, as previously observed. The time of onset of enhanced turbulence that is measured by the sensors on FLY and Autosub as the speed of tidal currents increases are in good agreement, as are their mean levels. Boils on the sea surface are identified using the Autonomously Recording Inverted Echo Sounder, version 2 (ARIES II), a two-beam upward-pointing side-scan sonar mounted on a rig resting on the seabed. The boils have mean horizontal dimensions of about 25 m. They are continually present when turbulence within the water column near the surface is large, typically when the rate of dissipation of turbulent kinetic energy per unit mass ℇ exceeds about 3 × 10-6 W kg-1, compared to the rate of about 1 × 10-7 W kg-1 near times of slack tide when no boils are observed. The top of a region with a relatively high ℇ gradually extends upward from the seabed as the tidal flow increases and, in stably stratified conditions resulting from tidal straining during ebb tides, can be detected in the ARIES II sonographs as it approaches the sea surface, although only after sound reflection from the sea surface. Upward-moving bursts of enhanced turbulence below the surface with horizontal dimensions of about 5–9 m in the direction of the tidal flow are identified in the AUV record in periods of high tidal flow using conditional sampling. The bursts result in slight upward displacements of the AUV from its set operational depth. Boils first appear at times before the region of generally enhanced turbulence reaches the surface: large localized upward-moving bursts appear to precede the main advancing region of small-scale turbulence. [ABSTRACT FROM AUTHOR]

The Southern Ocean hosts the formation of the densest layers of the oceanic overturning circulation and provides a climatically sensitive element of deep ocean ventilation. An oceanographic section across the eastern Scotia Sea occupied in 1995, 1999, and 2005 reveals significant variability in the deep and bottom waters of Southern Ocean origin. Warming (∼0.1°C) of the warm midlayer waters in the Scotia Sea between 1995 and 1999 reversed through to 2005, reflecting changes seen earlier upstream in the Weddell Sea. The volume of deep waters with potential temperature less than 0°C decreased during 1995–2005, though such a reduction was only clear between 1995 and 1999 at the southern end of the section. The abyssal waters of the eastern Scotia Sea changed circulation between 1995 and 1999, with the dominant point of their entry to the basin shifting from the south to the northeast; by 2005, the former route had regained dominance. These changes are best explained by interannual variations in the deep waters exiting the Weddell Sea, superimposed on a longer-term (decadal) warming trend. The interannual variations are related to changes in the strength of the Weddell Gyre, reflecting large-scale atmospheric variability that may include the El Niño–Southern Oscillation phenomenon. The Scotia Sea is the most direct pathway for dense waters of the overturning circulation emanating from the Weddell Sea to fill much of the World Ocean abyss. The regional changes reported here have the potential to affect the climatically significant ventilation of the global ocean abyss. [ABSTRACT FROM AUTHOR]

A form of linear, barotropic potential vorticity equation is derived for an ocean with a free surface, in which only one scalar variable appears (ocean bottom pressure, or subsurface pressure). Unlike quasigeostrophic or rigid-lid derivations, the only approximation made (apart from linearization) is that changes in the circulation must be slow compared with the inertial frequency. Effects of stratification are included, but only parametrically in the sense that density is treated as a given quantity or forcing term rather than a variable. [ABSTRACT FROM AUTHOR]

Barotropic to baroclinic conversion and attendant phenomena were recently examined at the Kaena Ridge as an aspect of the Hawaii Ocean Mixing Experiment. Two distinct mixing processes appear to be at work in the waters above the 1100-m-deep ridge crest. At middepths, above 400 m, mixing events resemble their open-ocean counterparts. There is no apparent modulation of mixing rates with the fortnightly cycle, and they are well modeled by standard open-ocean parameterizations. Nearer to the topography, there is quasi-deterministic breaking associated with each baroclinic crest passage. Large-amplitude, small-scale internal waves are triggered by tidal forcing, consistent with lee-wave formation at the ridge break. These waves have vertical wavelengths on the order of 400 m. During spring tides, the waves are nonlinear and exhibit convective instabilities on their leading edge. Dissipation rates exceed those predicted by the open-ocean parameterizations by up to a factor of 100, with the disparity increasing as the seafloor is approached. These observations are based on a set of repeated CTD and microconductivity profiles obtained from the research platform (R/P) Floating Instrument Platform (FLIP), which was trimoored over the southern edge of the ridge crest. Ocean velocity and shear were resolved to a 4-m vertical scale by a suspended Doppler sonar. Dissipation was estimated both by measuring overturn displacements and from microconductivity wavenumber spectra. The methods agreed in water deeper than 200 m, where sensor resolution limitations do not limit the turbulence estimates. At intense mixing sites new phenomena await discovery, and existing parameterizations cannot be expected to apply. [ABSTRACT FROM AUTHOR]

Gravity Recovery and Climate Experiment (GRACE) gravity data spanning January 2003–November 2005 are used as proxies for ocean bottom pressure (BP) averaged over 1 month, spherical Gaussian caps 500 km in radius, and along paths bracketing the Antarctic Circumpolar Current’s various fronts. The GRACE BP signals are compared with those derived from the Estimating the Circulation and Climate of the Ocean (ECCO) ocean modeling–assimilation system, and to a non-Boussinesq version of the Regional Ocean Model System (ROMS). The discrepancy found between GRACE and the models is 1.7 cmH2O (1 cmH2O ∼ 1 hPa), slightly lower than the 1.9 cmH2O estimated by the authors independently from propagation of GRACE errors. The northern signals are weak and uncorrelated among basins. The southern signals are strong, with a common seasonality. The seasonal cycle GRACE data observed in the Pacific and Indian Ocean sectors of the ACC are consistent, with annual and semiannual amplitudes of 3.6 and 0.6 cmH2O (1.1 and 0.6 cmH2O with ECCO), the average over the full southern path peaks (stronger ACC) in the southern winter, on days of year 197 and 97 for the annual and semiannual components, respectively; the Atlantic Ocean annual peak is 20 days earlier. An approximate conversion factor of 3.1 Sv (Sv ≡ 106 m3 s-1) of barotropic transport variability per cmH2O of BP change is estimated. Wind stress data time series from the Quick Scatterometer (QuikSCAT), averaged monthly, zonally, and over the latitude band 40°–65°S, are also constructed and subsampled at the same months as with the GRACE data. The annual and semiannual harmonics of the wind stress peak on days 198 and 82, respectively. A decreasing trend over the 3 yr is observed in the three data types. [ABSTRACT FROM AUTHOR]

Bottom topography, or more generally the geometry of the ocean basins, is an important ingredient in numerical ocean modeling. With the help of an adjoint model, it is shown that scalar diagnostics or objective functions in a coarse-resolution model, such as the transport through Drake Passage, the strength of the Atlantic Ocean meridional overturning circulation, the Deacon cell, and the meridional heat transport across 32°S, are sensitive to bottom topography as much as they are to surface boundary conditions. For example, adjoint topography sensitivities of the transport through Drake Passage are large in choke-point areas such as the Crozet–Kerguélen Plateau and south of New Zealand; the Atlantic meridional overturning circulation is sensitive to topography in the western boundary region of the North Atlantic Ocean and along the Scotland–Iceland Ridge. Many sensitivities are connected to steep topography and can be interpreted in terms of bottom form stress, that is, the product of bottom pressure and topography gradient. The adjoint sensitivities are found to agree with direct perturbation methods with deviations smaller than 30% for significant perturbations on time scales of 100 yr, so that the assumption of quasi linearity that is implicit in the adjoint method holds. The horizontal resolution of the numerical model affects the sensitivities to bottom topography, but large-scale patterns and the overall impact of changes in topography appear to be robust. The relative impact of changes in topography and surface boundary conditions on the model circulation is estimated by multiplying the adjoint sensitivities with assumed uncertainties. If the uncertainties are correlated in space, changing the surface boundary conditions has a larger impact on the scalar diagnostics than topography does, but the effects can locally be on the same order of magnitude if uncorrelated uncertainties are assumed. In either case, bottom topography variations within their prior uncertainties affect the solution of an ocean circulation model. To this extent, including topography in the control vector can be expected to compensate for identifiable model errors and, thus, to improve the solutions of estimation problems. [ABSTRACT FROM AUTHOR]

The topographical control of western boundary currents within a basin and zonal jets in a channel is investigated in terms of the potential vorticity (PV) and barotropic vorticity (BV: the curl of the depth-integrated velocity) budgets using isopycnic, adiabatic wind–driven experiments. Along the western boundary, the wind-driven transport is returned across latitude lines by the bottom pressure torque, while friction is only important in altering the PV within an isopycnic layer and in allowing a closed circulation. These contrasting balances constrain the geometry of the flow through integral relationships for the BV and PV. For both homogenous and stratified basins with sloping sidewalls, the northward subtropical jet separates from the western wall and has opposing frictional torques on either side of the jet, which cancel in a zonal integral for BV but alter the PV within a layer streamline. In a channel with partial topographic barriers, the bottom pressure torque is again important in returning wind-driven flows along western boundaries and in transferring BV from neighboring wind-driven gyres into a zonal jet. The depth-integrated flow steered by topography controls where the bottom friction alters the PV, which can lead to different PV states being attained for separate subbasins along a channel. [ABSTRACT FROM AUTHOR]

The theory of first-passage time distribution functions and its extension to last-passage time distribution functions are applied to the problem of tracking the movement of water masses to and from the surface mixed layer in a global ocean general circulation model. The first-passage time distribution function is used to determine in a probabilistic sense when and where a fluid element will make its first contact with the surface as a function of its position in the ocean interior. The last-passage time distribution is used to determine when and where a fluid element made its last contact with the surface. A computationally efficient method is presented for recursively computing the first few moments of the first- and last-passage time distributions by directly inverting the forward and adjoint transport operator. This approach allows integrated transport information to be obtained directly from the differential form of the transport operator without the need to perform lengthy multitracer time integration of the transport equations. The method, which relies on the stationarity of the transport operator, is applied to the time-averaged transport operator obtained from a three-dimensional global ocean simulation performed with an OGCM. With this approach, the author (i) computes surface maps showing the fraction of the total ocean volume per unit area that ventilates at each point on the surface of the ocean, (ii) partitions interior water masses based on their formation region at the surface, and (iii) computes the three-dimensional spatial distribution of the mean and standard deviation of the age distribution of water. [ABSTRACT FROM AUTHOR]

The response of a time-dependent atmospheric function close to a slope region is studied. A forced two-layered model over a sloping topography is examined with focus on the particular solution and interaction between the constructed barotropic and baroclinic mode. The water column responds with long periodic waves along the sloping bottom, and the solution includes a time-dependent Ekman transport in the upper layer. Despite its relative simplicity, the model produces a resonant response in the barotropic mode to a northward-propagating plane-wave wind stress field. The particular solution shows that a state close to resonance in the barotropic mode is given by a 1/λy line for the barotropic coefficient when plotted as a function of the forcing wave period and wavelength λy. From the Norwegian Meteorological Institute (NMI) Hindcast database, the calculated time series of the wind stress curl reveals a high energy level in the forcing period interval between 48 and 185 h. The Vøring Plateau slope has a natural oscillation period around 100 h for topographically trapped waves, and the model shows that an along-slope wavelength of 76 km will produce a resonant response in the water column for this wave period. Hence, when the forcing function is constructed as a sum of (infinitely) many Fourier components, topographic waves will be triggered along the Vøring Plateau slope for a northward-moving atmospheric pressure system. [ABSTRACT FROM AUTHOR]